An important requirement for the safe operation of a nuclear reactor is the need to keep the nuclear fuel below a certain critical temperature. In a boiling water nuclear reactor, this is accomplished by maintaining a sufficient inventory of coolant water in the reactor vessel and coolant flow through the fuel bundles. Insufficient cooling can cause damage to the cladding of the nuclear fuel rods. Thus, is it imperative that the coolant water level in the reactor vessel be continuously monitored.
In existing boiling water reactors, monitoring of the coolant water level is conventionally carried out by level gauges located in the downcomer region of the reactor. Although such gauges provide a reliable indication of the water level, the potential damage which may result if the water level were to fall too far makes redundant monitoring of the water level useful.
A lowering of the level of the coolant water in the reactor vessel may be due to a number of causes, e.g., through failure of the feedwater system, or if there is a leak through which coolant water is lost, or through a loss of steam. Unless the water inventory is replenished by an emergency or make-up water supply, the water level will continue to fall as coolant water continues to evaporate to form steam. Eventually, the upper portions of the fuel bundles will become overheated and the steam in contact with the overheated portions will become superheated.
Present day reactors incorporate number of safety features to indicate the onset of such a situation and to prevent it from occurring, or from progressing once started. For example, the aforesaid monitoring of the water level by means of level gauges and an emergency supply of coolant water are standard features in present day boiling water nuclear reactors. However, the detection of an abnormal condition in such a reactor is largely dependent on the proper operation of the level gauges. Thus, the potential consequences of the loss of a substantial amount of coolant water justifies redundant monitoring to detect the occurrence of an abnormal reactor condition.
In general, the presence of superheated steam in the reactor vessel indicates the existence of an abnormal reactor condition, usually due to a low water level in the reactor. A superheated condition is said to occur when the water and steam in the reactor vessel no longer exist in a saturation condition. In accordance with a commonly used technique for inferring the existence of a superheated condition in a boiling water reactor, and thus to provide redundancy of the monitoring process, a thermocouple, or the like, is exposed to the temperature of the steam in the steam exit line of the reactor. When the temperature sensed by the thermocouple exceeds a predetermined limit, a superheated condition, and hence an abnormally low water level in the reactor core, are assumed to exist. A disadvantage of this technique is that the high temperature of the steam and its radioactivity tend to deteriorate the dissimilar metals that make up the thermocouple and to accelerate the aging of the latter. Under these conditions spurious readings may be obtained, requiring frequent recalibration and accelerated replacement of the thermocouple.
To provide a more reliable determination of a superheated reactor condition, the equipment employed must be simple and it must not be degradable by the superheated, radioactive steam in contact therewith. Further, such equipment is preferably located outside the reactor, so as to be easily available for servicing. Finally, for true redundancy all monitoring performed with such equipment must be independent of the monitoring provided by the use of coolant water level gauges in the downcomer region of the reactor.